This invention relates to the field of optical assemblies, and more particularly, to interferometers for surface profiling, wherein the interferometer does not have to be manually aligned in order to achieve the profiling result.
Interferometers are old in the art. Interferometers are widely used in making accurate measurements of radiation spectra, distance measuring, equipment calibration and surface topography mapping.
Leading devices in the surface topography mapping field emanate from Zygo Corporation, of Middlefield Conn. Some basic U.S. patents directed to this inventive area, and owned by Zygo, include U.S. Pat. No. 3,844,660, issued Oct. 29, 1974 to Hunter, entitled METHOD AND APPARATUS FOR ALIGNING AN INTERFEROMETER MIRROR and U.S. Pat. No. 5,671,050, issued Sep. 23, 1997 to de Groot, entitled METHOD AND APPARATUS FOR PROFILING SURFACES USING DIFRACTIVE OPTICS
The prior art surface topography measuring devices use a Fizeau interferometer structure. Such a structure is shown in FIG. 1 of this document. The Fizeau interferometer creates an interference pattern between light beams reflecting off of a reference flat and a flat under test, wherein the two flats are parallel planes having an air gap (wedge) there between. The parallelism between the two flats, insures that the path taken by the light beam emanating from the light source will be identical for both flats (i.e., the beams reflecting off of both flats will overlap). Therefore, in order to create an interference pattern on a detector also found in the path of the beam, a parallelism between the two flats is needed. Essentially, a Fizeau interferometer functions by sending a beam of light, preferably a monochromatic (laser) beam of light, through a collimating lens, so as to align the beam for perpendicular translation and reflection off of the reference flat and the flat under test. The beams coming back off of the two flats then retranslate through the collimating lens to a beamsplitter, which reflects part of the beams to the detector. It is at the detector where the fringes are observed.
An interferometer surface topography apparatus directs the beams from the beamsplitter onto some type of conventional camera so as to produce an electronic image on a monitor for viewing of the surface topography of the tested flat.
Use of a Fizeau interferometer in these types of instruments has two major setbacks: (1) a fringe pattern of interference does not automatically appear, requiring manual manipulation (usually of the test flat); and (2) the instrument needs to be mounted on a vibration-isolated platform. The alignment process is time consuming and at times tedious, while the vibration-isolated platform is extremely costly.
Accordingly, it would be desirable to provide an alignment-free interferometer method and apparatus, which is not affected by vibration, in order to obtain surface typographies of flats under test.
In accordance with the invention, an improved method and apparatus for profiling surfaces is provided. The subject apparatus in addition to using a different construction then that of the normally used Fizeau interferometer, has the added inventive feature of the use of a retroreflector located at the end of the optical path of the beam reflecting off of the surface under test.
Accordingly, it is an object of the invention to provide an improved method and apparatus for profiling surfaces.
Still another object of the invention is to provide an improved method and apparatus for profiling surfaces not using a Fizeau interferometer structure.
Yet another object of the invention is to provide an improved method and apparatus for profiling surfaces, incorporating the use of a retroreflector assembly at the end of the optical path of the beam of light reflecting from the test object.
Other objects of the invention will in part be the obvious and will in part be apparent from the following description.
The invention accordingly comprises assemblies and methods of operation possessing the features, properties, relation of components and steps which will be exemplified in the products and methods hereinafter described, and the scope of the invention will be indicated in the claims.